AN/APG-70 Radar System
The heart of the F-15E's electronics suite is the AN/APG-70 radar developed by Hughes Aircraft Co. F-15C models have AN/APG-63, AN/APG-63(v1) and AN/APG-63(v2) (this latter being an ultramodern AESA type radar).
The AN/APG-70 is a highly reliable pulse doppler I/J band (8-20 GHz) multi-mode radar with improved maintenance features. It has both air-to-air and air-to-surface capabilities. It had been developed as part of the F-15E MSIP II programme (Multi-Stage Improvement Programme). It is sometimes called as an upgrade of the AN/APG-63 but in reality it is a brand new radar which has a couple of things common with the AN/APG-63. The AN/APG-70 was designed for greater reliability and easier maintenance. It is in fact 1/3 more reliable than the AN/APG-63 and has an 80 hour 'mean time between failure' rate.
The AN/APG-70 is built in the nose of the F-15E, behind the aerodynamically shaped nose radome, placed under and in front of the Cockpit. It uses a mechanically moveable main radar dish with high-gain and low-sidelobe array of emitters. The emitted radar beam covers a 120 degree (horizontally) arc in front of the jet, while the radar beam covers up to 8 bars - that is it can scan a line from right to left (that is called a bar), then drop down a bit and do another scan (bar) from left to right, etc. at a maximum of 8 bars. While changing bars the radar beam drops down approximately 1 degree at 40 nm or greater range, and it drops down approximately 2.5 degrees at 20 nm range. Antenna elevation can be adjusted, thus 'shifting' the vertical coverage up or down. The number of bars scanned can be set to different values: 1, 2, 4, 6 (default) or 8 bar scanning can be set.
The electronic section behind the bulkhead separating them from the antenna section of the AN/APG-70 consists of six major parts: the power supply, the radar data processor, the radar signal processor, the transmitter, the analog signal converter and the receiver/exciter. These are illustrated in the picture further below:
Power Supply: The power supply unit used in the AN/APG-63 had to be redesigned since the AN/APG-70's receiver/exciter requires slightly higher voltage than before.
Transmitter: This unit occupies the whole 'middle row' in the 3 level arrangement in the F-15E's nose and it is responsible for generating the radar energy that is emitted from the radar. Note that the AN/APG-70 is a coherent radar, which means that it is never really switched off during flight, even when the jet is 'silent' and not emitting radar energy. The radar signal source is always active, only the amplifier is turned on to emit radar energy or off to cease emitting.
Receiver/Exciter: The receiver/exciter boasts 32 channels and is the first unit that processes signals received directly from the antennas. The receiver first converts these analog signals to a lower frequency then passes them to the analog signal converter.
Analog Signal Converter: This unit digitizes analog signals received from the receiver/exciter. Digital signals are then processed further by the signal data processor.
Signal Data Processor (SDP): This unit gets radar input signals in digital format from the analog signal converter. The SDP is a computer which got significantly upgraded compared to that in the AN/APG-63 (5x faster and 10x more memory). The role of the SDP is to make a pre-processing of the received signals. It is connected to the Radar Data Processor via the MIL-STD-1750A data bus and together they utilize a modular parallel processing method to process and handle digitized input signals.
Radar Data Processor (RDP): This unit is a computer which is the heart of the AN/APG-70 system. It has 1 M memory (220 K for A/A modes, 110 K for A/G modes, 200 K for BIT, 64 K for UFC memory, the rest reserved for future upgrades), 10x more than that in the AN/APG-63. The basic role of the RDP is to tell target radar returns from false returns. Target returns are passed to the MPD processor, a unit in front of the power supply, which is responsible for visualizing target information on the radar page on the MPD's selected by the aircrew.
AN/APG-70 - General Features
Before discussing AN/APG-70 features one thing has to be stated firmly: since the AN/APG-70 gives a lethal edge to the F-15E both in A/A and A/G modes and since F-15E's are and will be intensively used in real combat situations throughout the world, the exact features of the AN/APG-70 are considered 'hot topic' and are classified. Any information on this site are from public domain searches and from outdated manuals hence should be considered as rough estimates - the actual modes and features of the AN/APG-70 may be entirely different from those described here.
The new AN/APG-70 unit has multiple bandwidths for high-resolution ground mapping using SAR technology. Several new radar modes were added, such as TWS (track-while-scan), which makes it possible to launch BVR missiles at separate targets simultaneously. Gate array technology enabled the AN/APG-70 to incorporate modes not available in earlier radars while providing greatly enhanced operational capabilities in other modes.
The increased processing power made it possible to implement Non-Cooperative Target Recognition (NCTR) technology, which provides the ability to distinguish more reliably between friendly and hostile aircraft.
The AN/APG-70 radar also had a Low Probability of Intercept (LPI) capability, which makes it able to detect and direct attacks on enemy aircraft without its emissions being easily seen by the enemy. This means that the radar can be quickly switched on to obtain a single-sweep synthetic aperture image of a target area, then rapidly switched off seconds later (or by another words, put in SNIFF mode), making it difficult for an enemy to pick up the emissions and track the F-15E's location and flight path. Putting the radar in SNIFF mode is absolutely essential during aerial refueling - otherwise the powerful microwave radiation emitted from the radar would cause severe injuries to the boom operator from such a short distance. For this reason a special security measure is installed into the radar which prohibits emitting radar energy when the aircraft is on the ground.
The radar map can be "frozen" on the screen, and updated periodically by new sweeps as the aircraft gets nearer to the target. The radar display terminals process the radar signals received and can provide a bird's eye view of ground targets. Roads, bridges, and airfields can be identified as far as 100 miles away, and as the F-15E nears the target image resolution becomes progressively sharper and smaller targets such as trucks, aircraft, and tanks can be distinguished.
AN/APG-70 - A/A Features
The AN/APG-70's can be put in A/A mode by taking command of an A/A radar display of one of the MPD's, in this mode its A/A features become available. In A/A mode, AN/APG-70 has several search modes of which only one can be active at a time. These modes can be selected by using the push buttons on the lower edge of the MPD/MPCD where A/A radar screen is selected (see picture below).
Targets picked up by the AN/APG-70 in either mode are displayed as "blips" on the A/A radar screen on the MPD/MPCD the aircrew had selected. The radar's software incorporates a special algorithm to filter out returns from ground moving objects. It has three sensitivity settings to filter out returns from objects moving slower than 45, 63 or 87 knots. This algorith comes also handy when returns from dropped chaffs (which slow down rapidly after dropping) should be filtered out - however the AN/APG-70 has other algoritms to filter out false chaff returns.
The radar beam operates by periodic scanning (at a normal scan rate of around 70 degrees per second), which means that it detects and re-detects the same target from time to time, so some time elapses between two consecutive detections of the same target. The target (especially fast moving targets) can move considerably within this time between successive detections, which means that the target blip displayed on the screen can lose its 'actuality' quite fast. This phenomenon is called target aging. Since the radar beam - due to the time it requires to take a full scan - cannot provide movie-like display of the aerial situation, target aging is handled by the MPD processor. The 'older' the target, the less bright its blip is on the A/A radar screen. If the pilot wishes so, the result of the past few scans can be displayed simultaneously on the screen (as a series of blips with ever decreasing brightness as the target ages), thus depicting the movement of the target
In A/A mode five different radar frequencies can be selected manually to avoid interference with the radars of other F-15E's of the same flight.
The AN/APG-70 features a 'mode' called NCTR (Non-Cooperative Target Recognition). Using NCTR the radar analyzes returns from the target and tries to identify the type and model of the target from certain tell-tale signs (returns from turbine and fan blades if the target flies at such aspect that the radar can 'have a look' at engines). NCTR utilizes a massive and continuously updated database of radar returns of different aircraft types and models - returns received from the actual target are searched in and compared to this database to have an ID of the target. NCTR is activated either automatically or manually and NCTR data are displayed on the radar screen. Since this is a highly advanced feature of AN/APG-70, most of it is classified.
AN/APG-70 has the following A/A modes:
Range While Scan (RWS) modes: The RWS mode provides all-aspect (nose-on, tail-on) and all-altitude (look-up, look-down) target detection. This is the most commonly used mode upon nearing a hostile environment. It is a good balance of wide volume, and fairly rapid scan. This mode is used to resolve (detect) multiple targets separated by less than the antenna beam-width, at long range. There are three RWS modes, depending on what pulse repetition frequency (PRF) is used for emitting radar energy. In RWSH mode high PRF is used, while in RWSM mode medium PRF is used. High PRF's are better to detect distant contacts with high closure rates with the risk of low or no-closure contact not showing up on the radar screen. Medium PRF's are not very good on long ranges since they are subject to clutter when receiving returns from long ranges, but they are useful for detecting medium-range low closure targets or targets which are below the radar water line. A good mixture of high and medium PRF's is realized in RWSI mode (interlaved RWS) when the radar emits eneregy alternating between high and medium PRF's as it scans through bars.
Range Gated High (RGH) mode: This mode uses a single PRF somewhere between those used in RWSH and RWSM modes. Returns are processed electronically to find low and high closure targets. This mode may not as accurate as RWSI, but can find certain targets more quickly. Note that this mode can detect targets up to a 160 nm range, but it is sensitive to altitude, it can be confused by ground clutter below 4000 feet.
Velocity Search (VS) mode: This mode is specifically for detecting medium and high closure targets, with the cost of not detecting low and no-closure targets at all. This mode displays targets on the radar screen by azimuth and velocity instead of by azimuth and range. Note that sometimes a fast moving part of the target (a turbine blade for example) can make VS mode detect a speed much higher than the actual speed of the target itself. This phenomenon is called Jet Engine Modulation (JEM).
Vector (VCTR) mode: The scan rate for this mode is half as normal, about 35 degrees per second. This means that a full scan takes twice the time, but the computer uses this time to make extra work, thus enhancing the detection of objects which have a low radar cross section (RCS). Lower RCS contacts are picked up from greater distances by using VCTR mode. This mode uses high PRF's only.
Track While Scan (TWS) modes: Track-while-scan means that the radar does its normal right-to-left, left-to-right scanning while it is actively tracking a couple of targets. TWS uses either high or medium PRF's. In TWS mode the radar beam covers an area much smaller than the maximal 120 degrees, but this way the target updates are much quicker. In takes around 2 seconds for the radar to complete a full scan. The arc and number of bars covered by TWS scan can be set to different settings: 'wide' (60 degrees with 2 bars), 'medium' (30 degress with 4 bars) and 'narrow' (15 degrees with 6 bars). See diagram below.
Since the radar normally can cover a 120 degree arc in front of the jet, it can be manually slewed in TWS mode, which means that the actually scanned area (15, 30 or 60 degrees) can be placed anywhere within the 120 degree limits.
The AN/APG-70 stores tracking information of up to 10 targets in TWS mode. This track record serves as a defense against loosing radar contact - if a target disappears from the screen, the system extrapolates from its track records and tries to predicts where the target should be next and then tries to re-acquire the target at the predicted spot. Additionally TWS provides speed and heading information to targets. Screen symbology of speed and heading information are small 'vector sticks' to point from the dot representing the target. The vector stick points to the heading of the target (the top of the screen meaning a heading of north) while the length of the stick illustrates the speed of the target. This is very useful for the aircrew to get a quick overall impression of the tactical situation. In TWS mode it is possible to put the designation cursor over a target to get important data of it (altitude, range, closure rate, heading, aspect angle, true airspeed) without locking up the target - this way triggering the target's RWR systems can be avoided.
There is a sub-mode in TWS which is called High Data Rate TWS (HDTWS). This mode halves the 2 second time needed for a full scan to 1 second at the price of halving the number of bars scanned. In HDTWS mode the radar scans either a 30 degree arc with 2 bars (called 'high data' TWS) or a 15 degree arc with 3 bars (called 'three-bar HD' TWS). Slewing is also possible in both HDTWS submodes.
The AN/APG-70 provides another sub-mode within TWS, other than HDTWS sub-modes. This sub-mode is designed to help target sorting, that is to make a difference between two or more targets that are flying very close to each other.
Single Target Track (STT) mode: If the pilot marks a single target for tracking, then the radar enters STT mode and begins tracking that specific target. This tracking uses a 3 degree mini-raster of radar energy centered on the target with very quick scans thus rapid target updates. If STT mode is entered from any of RWSI, RWSH, RWSM, RGH, VS or VCTR modes, then all other contacts disappear from the screen. If STT mode is entered from TWS or from HDTWS mode, all other targets remain on the screen. In STT mode exact data of this single target are are immediately available, such as altitude, range, closure rate, heading, aspect angle and true airspeed.
STT mode gives special aids to AIM-7 Sparrow usage (considered now obsolete). The AIM-7 Sparrow is a SARH (Semi-Active Radar Homing) missile, which means it has no radar of its own, but instead uses returns of radar signals originating from the launching F-15E for homing. Normally by entering into STT mode, the radar tries to switch to a medium PRF if it used a high PRF before. If the pilot places the AIM-7 Sparrow in priority, the radar tries to switch back to high PRF when the target enters into missile range. The AIM-7 Sparrow needs the high PRF for homing, since high PRF provides a more intensive signal exchange between the jet and the missile, hence makes homing easier. Since the SARH Sparrow requires continuous radar lock until impact, a numerical value is also displayed for the pilot giving the maximum angle (in degrees) that is allowed to steer without reaching radar gimbal limits, thus breaking lock for the Sparrow.
Dual Target Track (DTT) mode: This is the same as STT mode, but it tracks two targets simultanously. This mode supports the AIM-120 AMRAAM only (it is able to receive guidance from the F-15E in its non-active phase of flight) and provides no support for the AIM-7 Sparrow. Not that practically no one uses DTT mode, since TWS mode is much better from practically every aspects.
Auto Acquisition modes: All the modes above are for detecting targets while target designation is done manually by the pilot. Sometimes the situation dictates otherwise. To aid the pilot in snap-locking a target, there are five modes of the AN/APG-70, all of which are designed to acquire and designate the target automatically, within the limits of the given mode. These modes are the following:
Super Search (SS) mode: SS mode projects a 20 degree circle onto the center of the Head-up Display (HUD). The radar locks up the first target within 500 feet and 10 nm that enters this circle.
Boresight (BST) mode: BST mode projects a 4 degree circle onto the center of the HUD. The radar locks up the first target within 500 feet and 10 nm that enters this circle.
Long Range Boresight (LRBST) mode: LRBST mode projects a 4 degree circle onto the center of the HUD. The radar locks up the first target within 3000 feet and 40 nm that enters this circle.
Vertical Scan (VTS) mode: In VTS mode the radar beam covers a vertical area of about 7,5 degrees azimuth and of an elevation between 5 degrees and 55 degrees above the nose of the F-15E. The radar locks up the first target within 500 feet and 10 nm that enters this area. This mode is useful for targeting enemy aircraft in a turning fight.
Guns (GUNS) mode: Despite its name GUNS mode has nothing to do with the built-in gun of the F-15E. It is just a moderate range and wide scan auto ACQ mode of the radar. In GUNS mode the radar scans a 60 degree azimuth and a 20 degree elevation area (that is 30-30 degrees to the left and right and 10-10 degrees up and down). The radar locks up the first target within 3000 feet and 15 nm that enters this area.
Electronic Attack (EA) modes: In EA modes, the radar automatically reconfigures itself when inevitable signs of enemy jamming activity are detected. Facing enemy ECM activity, the radar tries to select special search and tracking modes which are the least sensitive to enemy jamming.
AN/APG-70 - A/G Features
AN/APG-70's A/G features are available for the aircrew when the radar is in A/G mode, which can be evoked by taking command of an A/G radar screen on one of the MPD's. In A/G mode, AN/APG-70 utilizes SAR technology to map the targeted area to produce a bird's eye view of it, where different ground features (as small as tanks, trucks, etc.) can be cleary distinguished. SAR (Synthetic Aperture Radar) technology requires to paint the target with radar energy from different ranges and azimuths. As a rule of thumb, the greater the size of the radar antenna, the better the resolution of the created map, which is often called as 'patch map' by fighter speak, or more oficially a HRM - High Resolution Map. The size of the F-15E radar antenna is much too small to produce a useable patch map, therefore creating a SAR image requires the jet to fly offset to the direction of the targeted area and make a couple of 'snapshots' from different azimuths and ranges. This way (aided by a powerful computer) a large sized radar antenna can be simulated by the jets movement and great resolutions can be achieved.
The AN/APG-70 also has a feature called PVU (Precision Velocity Update). By doing PVU the radar scans 8 sectors of terrain in front of the jet quickly in order to update/validate the velocity values used by the Inertial Navigation System (INS) to keep position. This simple technique makes it possible for the aircrew to designate the target from the generated SAR radar image.
AN/APG-70 has the following A/G modes:
Real Beam Map (RBM) mode: RBM mode displays radar returns from diferent terrain elements in front of the jet as an arc projected onto one of the MPD/MPCD's. On the image the F-15E is on the bottom of the display. The radar scans the terrain at a rate of 95 degrees per second. The range of the scan can be selected by the aircrew by choosing from the following values: 4.7 nm, 10 nm, 20 nm, 40 nm, 80 nm (this is a theoretical range of course, real ranges scanned depend on the LOS of the radar). Azimuth limits for the scan can also be set, available values are 50, 25 and 12.5 degrees to both sides of the F-15E (scanning 100, 50 and 25 degrees in azimuth, respectively).
Having an RBM displayed the WSO can select a point within the displayed arc to be the center point of a HRM to create. Then HRM can be created (see HRM mode, below) of the terrain section represented by the selected point.
The RBM image can be frozen by pressing the laser fire button on the Throttle Quadrant (either pilot or WSO). With the image frozen the results of successive radar scans will not be shown of course, giving reason for the aircrew to put the radar in 'SNIFF' mode during this time, that is prohibiting any radar emissions to leave the AN/APG-70. This not just reduces the chances of detection by enemy, but lets the WSO place his cursor more easily on the RBM image to produce a HRM (note, that many WSO's just don't freeze the RBM at all).
High Resolution Map (HRM) mode: A HRM can be produced practically from any area that is scanned and is displayed on the RBM image. There are some limitations however. Because of the reasons mentioned above, while producing a HRM image the F-15E should NOT fly directly towards the target. The greater the azimuth to the target, the less time it will take for the computer to produce a HRM (which is generally around 4-10 seconds) and the better the quality of the HRM will be - so the quicker the better. Flight level is also a factor. If the jet flies too low, the range to create a good HRM might not be enough. If the jet flies too high, the HRM might loose important details. The following diagram illustrates this.
The diagram depicts the AN/APG-70's 120 degree ground mapping coverage with arcs drawn representing quarters of the radar range set by the aircrew. The pinkish area right in front of the jet is called a blind zone: no HRM's can be created from within this zone, since azimuth offsets are not good enough here (theoretically saying, a SAR image is still possible to create from here, but it would require a huge amount of time, which is highly impractical in a tactical situation). The blind zone represents an area of 8-8 degrees azimuth to the left and right. The red areas are the best places to create a good quality HRM, these zones are between 30 and 50 degrees azimuth on both sides and between about 50% and 75% of the selected radar range (for example 20-30 nm if the range was set to 40 nm).
The HRM image covers a rectangular area with equal sides. The size of this area (called DW - Display Window) can be selected by the aircrew before letting the computer generate the HRM. Available DW sizes are 0.67 nm, 1.3 nm, 3.3 nm, 4.7 nm, 10 nm, 20 nm, 40 nm.
As it was mentioned before flight level is a factor for creating HRM's. The following table gives the maximum possible radar ranges at different altitudes flown (given in AGL) where HRM's can be created. The range values in the table are valid only over flat terrain - terrain obstacles such as hills or mountain ranges can of course limit the maximum radar ranges, since creating a HRM is a LOS dependent process.
|Altitude (AGL)||Maximum Range|
DW size setting can also affect the range from which the HRM can be created. As a rule of thumb, the smaller the DW size (that is the greater the SAR image resolution), the shorter the range the HRM can be produced from. The following table gives the minimum and maximum radar ranges a HRM of a given DW size can be produced from. The resolution of the HRM is also given in feet, meaning that objects smaller than the given resolution will be displayed on the HRM as if they were the size of the resolution itself (if they are displayed at all - this depends on their radar cross section).
|DW Size||min-max Radar Range||Resolution|
Note that the best possible resolution (which can be achieved from 20 nm, which is still quite a distance) is 8.5 feet - good enough to display even individual cars in a parking lot for example! As an illustration of how a real HRM looks like, here are some examples for real HRM images of different resolutions (all images below were made of an airfield).
Ground Moving Target (GMT) mode: Although the vehicles usually targeted (SAM's, trucks, tanks, etc.) do show up on the finest resolution HRM's, but they are still very difficult to pick up on a static HRM, especially if they are moving. To pick up moving ground objects the AN/APG-70 features a specific radar mode called GMT. GMT utilizes doppler shifts of ground returns to detect ground movements. Note that it does NOT detect the moving objects themselves (nor does it identify them), it just pinpoint a geographical area where movement is detected from. Movement detection is not possible beyond 32 nm, regardless of the radar range settings. GMT provides the same 'radar sweep' display as in RBM mode, but non-moving ground objects and terrain is filtered out, only moving objects are shown as small, bright crosshairs. This way they are easy to designate thus pass their position on to other targeting systems such as the targeting pod or the video seeker of a AGM-65 Maverick missile for example.
Interleaved Ground Moving Target (IGMT) mode: IGMT is the same as GMT with respect to detecting moving ground objects. It however superimposes target crosses over the usual RBM image, thus giving the WSO the added advantage of seeing the targets in their surroundings. Detection limits and designation methods are the same as in GMT mode.
|Text © 2006 Szabolcs Serflek|
Illustrations © 2006 Lutz Gretschel
Photos © US Air Force, www.fas.org
|Sources: Jane's F-15 Authentic Mode Manual, Steve Davies: "Boeing F-15E Strike Eagle|
All-Weather Attack Aircraft" (Airlife, 2003, ISBN 1 84037 378 4), Raytheon's official
website, The F-15E Strike Eagle Forum (SEF)